1. Field of the Invention
The present invention relates to a connecting device of a flexible printed circuit board. More particularly, the present invention relates to a connecting device in which a terminal portion of a flexible printed circuit board to be connected to a connector has an improved terminal portion, and especially to one suitable for use in a liquid crystal display apparatus.
2. Description of Related Art
Liquid crystal display apparatuses are widely used as displays in various electronic appliances such as cellular phones, video cameras, notebook personal computers, electronic dictionaries, and television monitors.
A liquid crystal display apparatus is composed of a liquid crystal display panel having liquid crystal sealed therein, a backlight unit for illuminating the display panel, and a control circuit for controlling them.
In recent years, various electronic appliances, in particular cellular phones, have come to be required to offer high performance and high functionality in combination with compactness, lightweight, slimness, and resistance to vibration, shock, and the like, while a large number of electronic components are built in such appliances at high densities and are connected together via printed circuit boards.
As such printed circuit boards, flexible printed circuit boards (hereinafter referred to as FPC boards) are often used, which are generally lightweight and easy to lay in complicated patterns.
FIG. 6 is a partial side view showing a conventional liquid crystal display apparatus employing such an FPC board.
This liquid crystal display apparatus 100 is provided with a liquid crystal display panel 103 and a backlight unit 108 for illuminating the liquid crystal display panel 103. The liquid crystal display panel 103 is provided with an array substrate 101 having thin-film transistors and transparent pixel electrodes formed in a matrix-like pattern thereon and a color substrate 102 having color filters, electrodes, and the like thereon. The array substrate 101 and the color substrate 102 are arranged to face each other, and a liquid crystal layer (unillustrated) is formed between them.
Of the array substrate 101 and the color substrate 102, the array substrate 101 has electrodes extending therefrom at one side edge thereof. These electrodes are connected to a flexible FPC board (hereinafter referred to as the driving FPC board) 104 so as to receive, from outside, a supply voltage and a video signal with which to drive the thin-film transistors.
The backlight unit 108 is provided with a light source 106 and a light guide plate 107 for directing the light from the light source 106 so as to illuminate the liquid crystal display panel 103. The light source 106 is connected to an FPC board (hereinafter referred to as the backlight FCP board) 109 so as to receive, from outside, a supply voltage.
The liquid crystal display panel 103 and the backlight unit 108 each have a predetermined thickness, and thus, when they are laid together, the driving FPC board 104 and the backlight FCP board 109 has a level difference between the respective connected end portions 104a and 109a in the direction of the thickness of the boards laid together. Thus, when the FPC boards 104 and 109 are laid together and bent toward the back face of the backlight unit 108, because of the level difference, the backlight FCP board 109 is bent with a sharper bending curve, that is, a smaller curvature (R), than the driving FPC board 104.
When the two FPC boards 104 and 109 are bent with different curvatures, bending them causes the following inconveniences. If the lengths of the driving FPC board 104 and the backlight FCP board 109 are adjusted when they are in a rectilinearly straightened state, and they are then bent, the backlight FCP board 109 so slides as to lose the place to go and thus so bends as to bulge upward.
Such bulging requires an extra space to be secured to allow for it when an apparatus is designed. This makes it difficult to make the apparatus compact, and makes interference with an obstacle likely.
By contrast, if the lengths of the two FPC boards 104 and 109 are adjusted when they are in a previously bent state, and the tip end of the backlight FCP board 109 is connected to a connector 105, straightening them from the bent state to a rectilinearly straightened state may cause the backlight FCP board 109 to be so pulled as to come off the connector 105.
A liquid crystal display apparatus has been proposed that is so devised as to overcome the above inconveniences (Japanese Patent Application Laid-open No. 2004-252339 (see FIG. 2 and paragraphs [0024] to [0029]). FIG. 7 is a front view of the liquid crystal display apparatus disclosed in this publication.
This liquid crystal display apparatus 110 is provided with a liquid crystal display panel 111 and a backlight unit 113 for illuminating the liquid crystal display panel 111. A driving FPC board 112 is connected to the liquid crystal display panel 111 at one side edge thereof, and, in a position away from a connected end portion 112a of the driving FPC board 112, a connecting piece portion 113a is provided that is connected to a light emission source (unillustrated). To this connecting piece portion 113a, a backlight FPC board 114 is connected. This backlight FPC board 114 has a central portion 116 of a main portion 115 thereof formed roughly in the shape of “S”.
As a result of the central portion 116 of the backlight FPC board 114 being formed in the shape of “S”, it can expand and contract in the length direction thereof. Thus, even when the FPC boards 112 and 114 are bent and straightened, the backlight FPC board 114 is prevented from bulging or coming off a connector 117.
A terminal portion at a tip end of such an FPC board is usually connected to a detachable connector (see Japanese Patent Application Laid-open No. H9-22760, FIGS. 3 and 4, and paragraphs [0002] to [0004]).
FIGS. 8A and 8B show the connecting device disclosed as prior art in this publication, FIG. 8A being a perspective view thereof, and FIG. 8B a sectional view in the direction perpendicular to the board surface shown in FIG. 8A.
This connecting device 120 is composed of a terminal portion 122 formed in an end portion of an FPC board 121 and a connector 123 into which the terminal portion 122 is inserted so as to be connected thereto. The connector 123 is formed as an elongate box-shaped housing 125 that has, formed in the front face thereof, an insertion slot 125a into which the terminal portion 122 is inserted and that also has, fitted in an inner portion inside the insertion slot 125a, a plurality of contacts 124. The connector 123 is further provided with pressing means 126 for fixing the terminal portion 122 in the insertion slot 125a. 
The connecting device 120 structured as described above works in the following manner. When the terminal portion 122 of the FPC board 121 is inserted into the insertion slot 125a, the contacts 124 of the connector 123 make contact with and thus conduct to the terminal portion 122 of the FPC board 121. In this state, when the pressing means 126 of the connector 123 is rotated in the closing direction, a pressing projection 126a thereof presses the FPC board 121 toward the contacts 124 as shown in FIG. 8B so that the terminal portion 122 of the FPC board 121 and the contacts 124 are pressed against each other and thereby connected and fixed together. In this way, the FPC board 121 is held on the board, and simultaneously the terminal portion 122 conducts, via the contacts 124 of the connector 123, to the printed circuit on the board.
Inconveniently, any of the FPC boards described thus far has the following disadvantages. The FPC board is formed as a strip-shaped member formed of very thin polyimide film or the like and having varying widths. Thus, the FPC board is extremely difficult to insert into an insertion slot, and requires skilled work for its connection to a connector. The reason is as follows. The FPC board, even in the terminal portion thereof, simply has contacts printed on a board in the form of film. Thus, the FPC board is, even at the tip end thereof, soft like, for example, a tip end of a piece of cloth, and tends to droop. Thus, even if an assembly worker, when inserting the FPC board into an insertion slot, attempts to do so after first determining the right direction, he takes time trying different insertion directions because he needs to vary the positioning of the tip portion, confirm the degree of insertion, and repeat those operations many times before achieving successful insertion into the insertion slot. The difficulty may be alleviated, as actually practiced, by bonding a reinforcing member on the face of the terminal portion opposite to the face thereof on which the contacts are provided with a view to making the terminal portion more rigid. Even when this is practiced, insertion is still considerably difficult.
The terminal portion has varying widths, and the above-described backlight FPC board has an especially small width, for example as small as about 2 mm. This makes insertion into an insertion slot more difficult.
Moreover, even when the terminal portion is inserted into the insertion slot, since the shape of the terminal portion is indefinite as described above, it is difficult to confirm whether the terminal portion has been inserted up to the right position, and, if not, bad contact may result. Such insufficient insertion may go undetected through electrical conduction inspection performed after assembly, causing disconnection resulting from vibration or the like during use, and thus leading to failure of an appliance.
Moreover, connection to a connector is achieved through work using human fingers or a tool. During this work, the fingers or tool may make contact with the contacts of the terminal portion, leaving contaminants, scratches, or the like on the surface of the contacts, and thus leading to bad contact.